1. Field of the Invention
The present invention relates to a ferrite carrier for an electrophotographic developer having a spherical shape, a high compressive strength, a high rate of compressive change, and an excellent strength against breaking due to stress received in a developing box when used for a developer, thereby preventing beads carry over and attaining a longer life, a method for producing the same, and an electrophotographic developer.
2. Description of the Related Art
A two-component electrophotographic developer used in electrophotography is constituted of a toner and a carrier; the carrier is mixed and agitated with the toner in a developing box; the toner is given a desired charge; and the charged toner is carried to an electrostatic latent image on a photoconductor whereby the developer is a carrier material to form a toner image. The carrier is, after having formed the toner image, held by a magnet and stays on a development roller, further returned to the developing box, again mixed and agitated with new toner particles, and repeatedly used in a certain period.
The two-component electrophotographic developer, different from a one-component electrophotographic developer, is one in which the carrier agitates the toner particles, imparts a desired chargeability, and has a function of transporting the toner, has good controllability in developer design, and is therefore widely used in the fields of full-color machines requiring high-quality images and high-speed machines requiring reliability and durability for image maintenance.
In such a two-component electrophotographic developer, ferrite particles made of Cu—Zn ferrite and Ni—Zn ferrite or the like have been used as a carrier in place of an oxide-coated iron powder and a resin-coated iron powder in order to obtain high-quality images. These ferrite carriers using the ferrite particles commonly have a spherical shape, and have many characteristics such as adjustable magnetic properties advantageous over the conventional iron powder carrier in obtaining high-quality images. Furthermore, a resin-coated ferrite carrier coated with any one of various resins using ferrite particles as the carrier core has adjustable specific volume resistance, and has enhanced abrasion resistance and durability or the like.
However, since the ferrite is a kind of ceramics, the ferrite after the ferritization reaction has high hardness. Adversely, the ferrite has the disadvantage that it is crushed by impact. When the particle size is reduced, a space between particles is also reduced, and the fusion of the particles is generated by high-temperature heating to complicate the sustainability of the spherical shape.
In recent years, in such two-component electrophotographic developer, the high-speed and full-color of development performance have been strongly required, and the reduction in the particle size of the carrier and toner has been required in order to obtain high-quality images in the requirement.
Referring to the toner, there have been proposed various toners having a small particle size and a sharp particle size distribution by a polymerized toner art or the like.
On the other hand, a formed magnetic brush is softened by reducing the particle size of the carrier, that is, by using the ferrite particles having a small particle size. Also, the specific surface area of the carrier is increased, and the quantity of the toner capable of being held is increased. As a result, larger effects have been expected for image quality such as image density, fog, toner scattering and tone reproduction.
However, when the particle size of the ferrite carrier is reduced, unfortunately, it becomes more difficult to sustain the spherical shape of the above-described ferrite particles.
Also, when the ferrite carrier with the toner is used as the developer, the ferrite carrier receives strong agitating stress in the developing box, and the carrier particles themselves may be broken. Since the particle size of the broken carrier particles are smaller than a carrier particle size permitted in designing the developer, the carrier particles are developed on the photoconductor with the toner, thereby causing image defects referred to as beads carry over. Since the particle size of the carrier particles has become smaller particularly in recent years, the broken carrier particles are turned into finer particles, which are apt to cause beads carry over.
User requirements for the image quality are increased year by year, and image defects caused by such beads carry over must be reduced as much as possible. Therefore, the carrier particles which are not broken as much as possible even if the carrier particles receives the agitating stress must be designed as one of measures against the beads carry over. As the core material of the carrier, ferrite particles are mainly used now. In the case of the ferrite carrier, according to, for example, Japanese Patent Application Laid-Open No. 9-6052, the breaking strength of the ferrite particles is desirably 5000 g/cm2 or more because such a level of the breaking strength makes the ferrite carrier highly abrasion resistant, and longer even in life.
A sintering step which is a final ferritization reaction step is mentioned as the largest factor for determining the strength of the soft ferrite particles. When the sintering temperature in the sintering step is low, and the growth of particle is not excessively progressed on the contact faces of the primary particles, the particles are apt to be broken to a primary particle size by agitating stress. Therefore, it is necessary to fire the ferrite particles at a temperature of more than a prescribed value in the sintering step in order to enhance the strength of the ferrite particles.
However, the higher the sintering temperature in the sintering step is, the growth of particle of the ferrite particles which are the carrier is progressed, and the strength thereof becomes higher, However, the influence of brittleness which is characteristic of ceramics is increased, and the ferrite particles become a so-called “hard but brittle” state where the ferrite particles are apt to be broken by the agitating stress. Therefore, it is necessary to optimize the sintering temperature in the sintering step, and suppress the influence of the brittleness while maintaining the strength of the ferrite particles themselves to more than a prescribed value in order to enhance the strength of the ferrite particles against the agitating stress in the developing box.
Thus, the ferrite particles (carrier) having strong strength and proper brittle balance, withstanding agitating stress, preventing beads carry over and attaining a longer life have been required.